1. Technical Field of the Invention
The present invention relates generally to a compact structure of a spark plug for internal combustion engines which may be employed in automotive vehicles, cogeneration systems, or gas feed pumps, and more particularly to such a spark plug designed to ensure a desired heat range.
2. Background Art
Spark plugs are usually used in internal combustion engines such as ones mounted in automotive vehicles. The output of the engine is increased or the fuel consumption rate is improved by increasing the diameter of intake valves or exhaust valves leading to an intake manifold or an exhaust manifold of the engine. The cooling system is also improved by increasing the size of a water jacket as needed. This requires the need for downsized spark plugs in which a thread formed on a mounting shell has a diameter of M12 or less, as specified in JIS. The downsizing of the spark plugs requires thinning the center electrode thereof.
Usually, the spark plugs are required to minimize the overheating of the tip of the center electrode to avoid the pre-ignition. To this end, Japanese Patent First Publication No. 5-13147 teaches use of a center electrode 9, as illustrated in FIG. 9, made up of a high thermal conductive Cu-made core 91 and a Ni-made outer layer 92 to improve the degree of transfer or dispersal of heat from the spark plug (i.e., the heat range).
The whole of the Cu-made core 91 is, however, disposed inside the Ni-made outer layer 92, thus resulting in a lack in transferring thermal energy from the outer surface of the center electrode to a porcelain insulator surrounding the center electrode in terms of improvement of the heat range of the spark plug.
The thinning of the center electrode 9 requires decreasing the diameter of the Cu-made core 91. Such decreasing results in a reduction in thermal conductivity of the center electrode, which leads to a decrease in the heat range of the spark plug. The thinning of the center electrode 9 may also be achieved by decreasing the thickness of the Ni-made outer layer 92, but however, Cu is higher in thermal expansion, thus causing the Cu-made core 91 to expand to do physical damage to the Ni-made outer layer 92 when the temperature of the center electrode 9 rises.
The improvement of the degree of dispersal of heat from the spark plugs is typically achieved by decreasing the length of a leg (also called a nose) of the porcelain insulator (i.e., the distance between the tip of the porcelain insulator and a portion of the porcelain insulator born by a mounting shell of the spark plug. This, however, encounters the drawback in that the antifouling ability of the spark plug degrades.